Historically, printing has been done by applying ink to a specially configured key or carrier and mechanically impacting the key or carrier on a recording medium such as paper to form an impression of the carrier. More recently, non-impact printing devices have been developed, where intelligent patterns (alphanumeric characters, common graphics and the like) are deposited on a recording medium. Non-impact printing devices utilize a variety of methods of forming the intelligence patterns including chemically active and chemically inert processes, using either fluids or solids as the marking or printing medium, and requiring either specially treated recording media or untreated recording media.
It has been known to print by depositing discrete droplets of printing fluid on a recording medium in a predetermined pattern. Previous attempts to achieve such a method of printing utilize a continuous stream of fluid which separates into droplets which are charged and electrostatically deflected so that they form the desired pattern on the recording medium. Such methods produce acceptable resolution typically only when the charge per unit mass is accurately controlled for each drop. This can be accomplished in two ways: the droplets are either given equal charge per unit mass and then deflected by an electrostatic field whose intensity is controlled by the input signal, or the droplets are given a charge per unit mass according to the input signal and then deflected using a constant electrostatic field. Existing embodiments of both of these methods require that the fluid droplets be substantially uniform which has proven difficult to achieve. Once the stream of uniform droplets has been attained, it is usually necessary to provide voltages in the range of 2,000 to 10,000 volts for the electrostatic field. Such voltages are difficult and expensive to produce and control. Also, the process of charging the droplets themselves sometimes causes electrolysis of the printing fluid, creating corrosive bi-products which may cause electrode deterioration.
In an effort to obtain droplets of uniform size, different methods have been applied in the prior art. First, the printing fluid is delivered to a nozzle at sufficient pressure to assure that a continuous jet of fluid is issued from the nozzle. The jet stream is separated into droplets by using radial oscillations of vibration induced in the nozzle itself by means of magnetic drivers or piezoelectric crystals. Vibrations cause regularly spaced varicosities in the ink stream, aiding the natural tendency of the stream to separate into droplets and making the ensuing droplets more uniform than would otherwise occur. Such devices typically provide for having a plurality of ink streams issuing from a row or rows of nozzles and require a support means for the nozzles which contains the ink channel and a resonant acoustic cavity such as shown in U.S. Pat. No. 3,373,437. The material must be rigid in order to hold the nozzles fixed for accurate printing and implies a metallic material which implies it has a high acoustic impedance. For such a device to be effective, the resonant acoustic cavity within the support means must typically include the ink channel itself to be excited by either plates or pistons which in turn are excited by a piezoelectric transducer.
Another approach to droplet formation utilizes printing fluid delivered to the nozzle under sufficient pressure to form a meniscus at the nozzle not high enough to produce flow through the nozzle. In this method, the fluid is drawn from the nozzle electrostatically in a ray-like jet which is then deflected electrostatically as desired. The electrostatic field which draws the jet of fluid from the nozzle is constant, producing a continual stream of printing fluid. The stream breaks into a succession of droplets with essentially uniform mass and charge. A time varying electrostatic field controlled by the input signal is then used to deflect the droplets as required for the formation of alphanumeric characters. The foregoing printing processes and mechanisms make use of a continuous flow of printing fluid, with the flow to be diverted to a reject basing or collector whenever no characters are patterns are to be printed. This may result in a more complicated system for hindering the flow of printing fluid than would otherwise be desired.
In another type of device which is shown in U.S. Pat. No. 4,331,964, a piezoelectric transducer is employed to create acoustic waves in a solid rubber cavity.
In the type of device such as shown in the U.S. Pat. No. 4,331,964, it is effectively a dual cavity system in which the ink channel is one cavity which in turn receives acoustical energy from another rubber filled cavity which is cylindrical, and is excited by a cylindrical piezoelectric. A membrane is specified between the two cavities.
Ink or rubber has lower acoustic impedance than the support that forms the cavity (for water-based ink vs. steel, the ratio is about 1/25). As a result, an acoustical standing wave is set up in this cavity by the transducer which vibrates typically at a frequency in the range of 50-150 KHz. In order for the breakup of jet streams to be uniform, the standing wave pattern must be uniform along the jet array.
A further critical element of the design system is the piezoelectric transducer which is used to produce uniform vibrations into the acoustical cavity. If the vibrations are not uniform, the acoustical standing wave pattern will most likely also not be uniform. For example, U.S. Pat. No. 2,716,708 is a device for launching ultrasonic waves. Grooves are cut into the piezoelectric material to form a linear array of elements. The elements vibrate in antiphase. Therefore, this device does not function properly for use as an inkjet even if the elements vibrated in phase due to the excessive relative width of the array. Another patent showing a transducer is U.S. Pat. No. 4,550,606. This patent shows an ultrasonic transducer array with controlled excitation pattern. Similar disclosures are found in U.S. Pat. Nos. 4,095,232 and 4,138,687. However, in all these cases, the application is directed to generating ultrasonic compressional waves in materials (human tissue) with scattering centers for the purpose of imaging the scattering centers from their echoes. Such a device as shown in the figures of the patents would not function in an inkjet device. The piezoelectrics shown therein would not generate the necessary amplitude signals with uniformity of amplitude of vibration because the device is designed to generate vibrations several wavelengths away from itself. Near the device, uniformity of the vibration would not be adequate for an inkjet printer.